To take over a Mac, the attacker needs physical access. He then plugs in Thunderstrike-enabled hardware — a modified Thunderbolt-to-Ethernet dongle in the demo — and reboots the Mac, triggering the process that replaces the firmware with malicious code. That’s it.
While all Macs are technically vulnerable to the Thunderstrike attack, few readers face any immediate risk. The attack is highly targeted — someone needs both physical access to your Mac and time to reboot it and reinstall the firmware.
As always when it comes to Macs and security, Rich Mogull is my go-to guy. He writes well and clearly about the subject and I trust his opinion and advice.